The present invention relates to a system and method for controlling the heat exchange function in a patient temperature control system and more particularly to controlling the heat exchange in a manner which avoids temperature overshoot and undue temperature oscillation.
The use of contact pad systems for selectively cooling and/or heating bodily tissue is known. In such systems a fluid, e.g. water or air, is circulated through one or more pads to affect surface-to-surface thermal energy exchange with a patient. One highly effective contact pad and related system is disclosed in U.S. Pat. No. 6,197,045, hereby incorporated by reference in its entirety. As noted in the ""045 patent, the ability to establish and maintain intimate pad-to-patient contact is often of key importance to fully realizing medical efficacies with contact pad systems.
In this later regard, the effect of temperature on the human body has been well documented. Elevated temperatures, or hyperthermia, may be harmful to the brain under normal conditions, and even more importantly, during periods of physical stress, such as illness or surgery. Conversely, lower body temperatures, or mild hypothermia, may offer some degree of neuroprotection. Moderate to severe hypothermia tends to be more detrimental to the body, particularly the cardiovascular system.
Temperature management, or thermoregulation, can be viewed in two different ways. The first aspect of temperature management includes treating abnormal body temperatures, i.e. cooling the body for elevated temperatures, or warming the body for lowered temperatures. The second aspect of thermoregulation is an evolving treatment that employs techniques that physically control a patient""s temperature to provide a physiological benefit, such as cooling a stroke patient to gain some degree of neuroprotection.
Hypothermia may occur for a variety of reasons, including exposure to cold environments, brain injury, or complex surgical procedures. During surgery, a patient typically experiences mild hypothermia as a result of the effect of general anesthesia on the body""s thermoregulatory system and prolonged exposure of internal organs. Mild hypothermia in the medical or the surgical patient has been thought to prolong the time to extubation, contribute to coagulopathies, increase the chance of infection, and increase cardiac demand as a result of shivering.
Hyperthermia may occur as a result of systemic inflammatory response, sepsis, stroke, or other brain injury. While the mechanism of the effect of the hyperthermia on the brain is not clearly understood, there is evidence to indicate that even mild increases in temperature may contribute to neurological deficits. Hyperthermia also increases the metabolic rate and may deplete energy stores in the body.
In view of the foregoing, it may be appreciated that recognized medical applications for contact pad systems are ever-increasing. By way of example, cooling pad systems may be utilized in early therapy to reduce neurological damage incurred by stroke and head trauma patients. Additional applications include selective patient heating/cooling during surgical procedures such as cardiopulmonary bypass operations.
As these and other medical applications have evolved, the present inventors have recognized the desirability of enhancing the predictability, responsivity, flexibility and portability of thermal exchange pad systems. More particularly, while known heating/cooling contact pad systems have proven effective for many applications, the present inventors have recognized that additional performance objectives and potential applications can be realized via the implementation of further improved control systems and associated control methodologies.
The inventors have recognized that it would be advantageous to configure a temperature control system which provides precise control of various heat exchange devices employed therein. The inventors have further recognized that the system may include a specially configured controller which employs various temperature control modeling which significantly reduces temperature oscillation and overshoot.
Described herein is a temperature controller for use in patient temperature control, system wherein the controller which is electrically connectable to a plurality of temperature sensors which monitors temperature at the inlet and outlet of the system and a flow meter which monitors flow rate of water to and from temperature control pads positionable on a patient for providing temperature control. Signals received at the controller indicate flow rate of fluid circulating through the system, the output temperature of the fluid circulating to at least one temperature control pad and the input temperature of fluid circulating from at least one temperature control pad.
During operations of the temperature control system, the controller is configured to process the received signals from the various sensors and calculate a number of control terms. The control terms are further employable to calculate one or more power signals which are transmittable to one or more heat exchange devices. The control terms may include first control term which is proportional to a first difference in temperature of fluid circulating in the system for fluid between the inlet temperature and selected set point. A second control term also employable by the controller may be proportional to a difference between the outlet temperature and a selected temperature set point. The system controller may further employ the first and second terms to generate a power signal which is transmittable to one or more of the heat exchange devices. In response, the heat exchange device may then affect temperature of the circulating fluid accordingly.
In one configuration of the invention, the heat exchange devices employable by the system may include a first heat exchange device specially configured for heating a fluid in response to a power signal. A second heat exchange device may be configured to cool the circulating fluid in response to receipt of the power signal. One or more auxiliary pumps may be employed for moving fluid past a heat exchange device.
In another configuration of the invention, the first control term may be modeled to be a theoretical heat transfer rate required to raise or lower the circulating water from the measured inlet temperature to a selected temperature set point. More particularly, the first control term may be calculated through a combination of a gain term, a measured flow rate, a provided flow offset, as well as the temperature difference described above.
The second control term may be specially configured to control the rate of change of temperature as the measured temperature at the outlet reaches the temperature set point. Specifically, issues that are addressed by the second term are oscillation and overshoots of the outlet temperature about a set point. According to another configuration of the invention, the second control term may be based on a modified proportional, integral, and derivative (PID) controller. Included as part of the PID control algorithm may be a modified integral and modified derivative terms. Also employable is a variable gain which is related to the measured flow rate of the fluid through the system.
With regards to the modified integral term, it may be approximated by a summation of measured errors over time. The error is the measured temperature difference between the outlet temperature and the temperature set point. The measured error may be employable in a controlled fashion. For example, a current error may be used in the summation if the error is decreasing too slowly or increasing at any rate. Integration may be further restricted such that the integral is not changed if the magnitude of the error is larger than a prescribed value. Minimum and maximum value of the integral term may be further limited so that it does not become to large a portion of the overall control of the temperature exchange devices.
With regards to the modified derivative term, this may be calculated through determining the slope of a xe2x80x9crunning least squaresxe2x80x9d line fit to the most recent N data points (errors). The magnitude of the modify derivative term may be also limited in certain situations to prevent response to noise.
According to the invention described herein, the system may be further configured to operate in one or more different modes depending on a measured temperature either at the inlet or outlet. For example, temperatures which are too high or too low may be dangerous to a patient. As such, the control model described herein may be configured to respond differently at certain temperatures. In one mode, if a temperature is detected to be too high, the modified integral term may be changed to increment at a faster rate. This may be done by multiplying the modified integral term by a weighting factor. Further, the modified derivative term may be disabled if the temperature is dropping.
According to the invention described herein the controller may be configured as part of a temperature control system. The temperature control system may include one or more temperature sensors which monitor temperature at the inlet and outlet of the system. The outlet temperature sensor is specially configured to measure temperature of water flowing from any reservoir within the system to temperature control pads which are positionable on a patient. The inlet temperature sensor measures fluid temperature flowing from the temperature control pads to the system described herein. The system may further include one or more reservoirs for holding the circulating fluid and one or more pumps for circulating the fluid through the system at a selected rate. Further included in the system may be at least one auxiliary pump configured for pumping the fluid through the first or second heat exchangers to affect the heat exchange between the medium located in the heat exchanger and fluid. Depending on the system configuration, the rate of fluid flow through the auxiliary pump may be the means of controlling temperature. Further in connection with the controller may be a user interface device, through which a system user may manually enter values to be employed by the system such as temperature set points and minimum and maximum temperatures. Programmed sequences for controlling temperature may also be entered. For example, during surgery, controlled cooling or heating may be employed during different parts of the procedure. These sequences may be preprogrammed such that they may be automatically or manually initiated.
With regards to system operation, the temperature control pads may be connected to the temperature control system and a temperature control set point identified prior to performance of a surgical procedure. Once the procedure is begun and the temperature control system is initiated, periodic readings are taken at the inlet and outlet temperature sensors as well as from the flow meter. For each of these periodic readings, values for the first and second control terms are calculated. As was discussed above, first control term is proportional to a measured error at the temperature inlet between the measured fluid temperature and the set point. The second control term is proportional to an error between the outlet temperature and the set point.
Once the first and second control terms are calculated a power signal is generated and transmitted to the heat exchange device identified to provide the desired type of heat exchange. As the temperatures are being monitored it may be detected that the measured temperatures in either the outlet or inlet exceeds a specified range of temperatures. In this situation, the controller automatically initiates an alternative mode of operation wherein the second term is incremented at a faster rate. Once the temperature begins moving in the direction of an acceptable range, the derivative term may be set to zero so as not to slow the adjustment process. Incrementing the second term at a faster rate is continued until it is detected that the outlet temperature is back within the desired range. At this point the controller may initiate the normal mode of operation. During operation, as part of a pre-programmed sequence, one or more temperature set points may be identified whereby the system alters the fluid temperature to the desired temperature set point.